This invention relates to clock recovery for optical networks. Optical network systems often employ a clock that generates periodic, accurately spaced signals for various purposes including synchronization of interconnected equipment and the regulation of processor operations.
However, many physical mechanisms prevent the accurate optical transmission of a clock signal. For example, spontaneous emission noise changes the clock pulse shape; timing jitter changes the timing between the clock pulses; signal attenuation along the fibre changes the clock pulse power; and the like.
As a result of such optical signal degradation, clock pulses need to be retimed, reshaped and re-amplified at strategic points along an optical network in order to restore degraded clock pulses to their former quality (optical 3R regeneration). This is referred to as clock recovery.
There are various forms of clock recovery presently employed. One method uses the tuned circuit. Transition in the digital signal is used to stimulate an impulse response from a tuned circuit, which then oscillates with decaying amplitude until it is re-stimulated. However, noise in incoming signals may produce extraneous impulses that stimulate the tuned circuit and thereby corrupt the clock signal. Further, as its frequency of operating cannot change, the tuned circuit cannot handle jitter.
Another form of clock recovery employs a phase locked loop (PLL), for example, Tong et al., IEEE Photon. Tech. Lett., vol. 12, pp. 1064-1066, 2000. A PLL comprises a phase detector, a loop filter amplifier and a voltage-controlled oscillator. However, employing a PLL adds additional complexity and cost to the system. Further, it is unable to exactly reproduce or regenerate an incoming signal, resulting in deviations from synchronicity.
One method of optical clock recovery used employs an external cavity resonator in a receiver laser carefully tuned to a specific frequency, for example, Mathason and Delfyett, J., Lightwave. Tech., vol. 18, pp. 1111-1120, 2000. Weak injection of a signal with a frequency component at the tuned frequency results in the external cavity configuration becoming locked to the tuned frequency component of the incoming signal. While such systems are able to initially provide synchronicity, phase shifting or thermal drift arising from temperature fluctuations eventually causes the external cavity resonator frequency to vary with the result that such a system loses coherence.
Accordingly, it is desirable to have a clock recovery system that does not substantially rely on electronic components in the clock recovery process, that is not affected by thermal drift and that allows the system to remain synchronized as long as there is an active connection between components.